Focal segmental glomerulosclerosis (FSGS) is a progressive proteinuric kidney disease and the third leading cause of end-stage renal disease in the US. Genetic studies demonstrate that FSGS is largely a podocyte disease. The molecular cause of FSGS is unknown in over half the cases and effective treatments remain elusive. We performed whole exome sequencing (WES) in two siblings with FSGS and identified a shared nonsense mutation in MYO9A, which encodes an unconventional myosin not previously associated with renal disease. This proposal overall objective is to determine whether human MYO9A mutations cause FSGS, define the molecular pathogenesis and identify novel therapeutic targets. MYO9A is a motorized Rho-GAP that controls collective cell migration and negatively regulates RhoA activity at cell-cell junctions. RhoA modulates podocyte actin dynamics and is important to maintain glomerular permeabllity, but podocyte RhoA constitutive activation results in proteinuria and glomerulosclerosis in mice. Preliminary data show that MYO9A is expressed in kidney podocytes and regulates their size. The MYO9A mutation identified in FSGS patients predicts a truncated gene product that lacks the RhoA-GAP domain. To assess the functional consequences of this mutation in vivo we generated Myo9A mutant mice using CRISPR-Cas9 mediated gene editing, which developed proteinuria, foot process effacement and glomerulosclerosis. We hypothesize that MYO9A mutations can cause FSGS by disrupting MYO9A podocyte Rho-GAP activity. We propose to evaluate the mutant MYO9A function in vivo and in vitro, and to identify additional MYO9A mutations in large FSGS/ SRNS cohorts. Specific aims are: 1) Establish whether MYO9A mutation leads to FSGS/NS in mice by completing the phenotypic analysis of mutant Myo9A mice and testing whether Rho inhibition modulates the mutant Myo9A phenotype and renal disease progression. 2) Elucidate the molecular pathogenic mechanism of the novel FSGS-associated MYO9A mutation by evaluating MYO9A signaling and function in normal human podocytes vs. MYO9A knockdown and mutant MYO9A podocytes using biochemical and cell assays. 3) Identify MYO9A mutations in FSGS-Clinical trial patients by WES, and identify additional patients with MYO9A mutations in large SRNS/FSGS cohorts. These studies will advance our mechanistic understanding of human FSGS through cells and mice carrying the novel MYO9A mutation, generated by gene editing. This translational strategy holds promise to deliver personalized, mechanistically based therapies for glomerular diseases within relatively short time span.